We found a 1000-fold increased risk of sunlight induced cancers of the skin and eyes in XP patients along with a 50 year reduction in age of onset of these cancers, providing compelling evidence that DNA repair plays an important role in cancer prevention in the general population. We ascertained and intensively examined 80 XP patients and 32 TTD patients at NIH (39 XP patients and 18 TTD patients since the last site visit) and are collaborating with researchers in Israel, Turkey, Europe and Japan to study additional patients. We are studying clinically normal family members of XP patients to determine if XP heterozygotes have increased cancer risk. We have established several hundred carefully documented cell lines and contributed them to cell banks for use of the general scientific community. Our laboratory has become a major center in the US for basic, clinical and translational expertise concerning DNA repair related disorders. We have developed host cell reactivation assays for measuring DNA repair and mutagenesis at the molecular level in human cells. We have now identified mutations in all 8 DNA nucleotide excision repair (NER) genes (XPA, XPB, XPC, XPD, XPE, XPF, XPG, and TTDA) and in the error-prone polymerase, pol eta in more than 150 XP and TTD families. In a study of 16 XP-C families we found that most of the XP-C patients had mutations leading to premature stop codons. We found low levels of XPC mRNA in XPC patients while their obligate heterozygote parents had an intermediate level of XPC mRNA between the patients and the normal controls. We found two XP-C families from Turkey that had severe or mild disease that correlated with mutations in two different splice lariat branchpoints (LBP). These LBP mutations resulted in no measurable XPC mRNA or a low (3%) level of XPC mRNA, respectively. This indicates that a) low levels of normal XPC mRNA may provide some protection against cancer and b) reduced levels may increase cancer risk (e.g. potential risk to heterozygotes). We identified new mutations in the XPB gene in cells from XP patients with and without the XP/Cockayne syndrome complex. We found a wide spectrum of mutations in pol eta in cells from 10 XP variant families in America, Europe and Asia. We are continuing to compare the genotype to the clinical features of XP patients and to characterize the progressive neurologic degeneration that occurs in some XP patients because it may be a model for more common neurodegenerative disorders. In contrast to the profound environmental influence on XP, TTD is a disease of altered development. We performed a comprehensive literature review and identified reports of 112 TTD cases. We found a wide variety of clinical symptoms in many organ systems including an unsuspected 20-fold increase in mortality in children under age 10 years primarily due to infections. There were reports of increased fetal developmental abnormalities and pregnancy complications in the mothers of TTD patients providing evidence of DNA repair gene function in human fetal growth and pregnancy. Using localized UV irradiation we were able to show a consistent difference in the pattern of movement of NER proteins in cultured cells from XP or TTD patients with different XPD defects. In XP cells at late times after UV, the NER proteins persisted at sites of unrepaired DNA damage. In contrast, in TTD cells the NER proteins did not persist. These differences suggest a mechanism that may explain the difference between the increased cancer susceptibility in XP patients and the absence of increased cancer in TTD. While skin cancer is the most common cancer occurring in the US, and rates of melanoma, the most dangerous skin cancer are rapidly increasing, there remains controversy as to the role of UV in induction of cutaneous melanomas. Because of their high frequency of melanomas, XP patients are a unique resource to study the pathophysiology of melanoma. We used laser capture micro-dissection to remove melanoma cells from tissue blocks of XP melanomas and sequenced the PTEN tumor suppressor gene. We found a high frequency of UV-type mutations in 59 XP melanomas. These data provide evidence for a direct effect of UV in the development of melanoma. In one of the first controlled studies of effective chemoprevention of skin cancer in humans, we found that, in XP patients, oral 13 cis retinoic acid was very effective in preventing skin cancers, although all patients had toxicity. We are continuing to follow these patients to determine the long term effects of therapy.